A radar apparatus is disclosed in U.S. Pat. No. 5,805,527 (corresponding to JP-A-H9-236661). This radar apparatus emits a laser light and receives a reflected light from an object. This radar apparatus measures a distance to the object based on a time period between an emission time when the laser light is emitted and a reception time when the reflected light is received. Therefore, it is necessary that the radar apparatus detects the reception time accurately.
In order to detect the reception time, the radar apparatus converts the reflected light to a received signal whose magnitude corresponds to an intensity of the reflected light. For example, it is possible that a timing when a rising of the received signal reaches a predetermined threshold value for avoiding an influence of noise is regarded as the reception time. However, the slope of the rising and falling of the received signal depends on the intensity of the reflected light, which changes depending on an intensity of the emitted laser light, a reflectance of the object and the like. Accordingly, the reception time changes according to the intensity of the reflected light even when the distance to the object is the same.
Therefore, the above radar apparatus performs the following processing in order to detect the reception time tp.
(1) A time t1 when the rising of the received signal reaches a predetermined threshold value for avoiding an influence of noise is detected.
(2) A time t2 when the falling of the received signal reaches the predetermined threshold value is detected.
(3) A mean time tc (=(t1+t2)/2) of t1 and t2 is calculated.
(4) A time period Δt (=t2−t1) between t1 and t2 is calculated.
(5) A correction amount Δα for correcting tc is determined according to the relationship between Δt and the correction amount Δα. The relationship is determined based on an experimental result or the like in advance.
(6) The reception time tp is calculated by subtracting AΔα from tc.
According to the above processing, the reception time tp can be substantially set to a timing when the received signal reaches its peak. Therefore, the reception time tp is not influenced by a difference of the intensity of the reflected light.
However, the received signal may saturate when the laser light with high intensity is emitted to detect the object at a long distance and the object at a short distance or the object with high reflectance to the laser light exists. In this case, the correction amount Δα becomes too large and the reception time tp cannot be calculated accurately. Therefore, an accuracy of detection of the object is lowered.